Method for forming severance lines



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ROBERT H. DOWN/ BY mstclaa A T TORNEV April 5, 1966 R. H. DOWNIE METHOD FOR FORMING SEVERANCE LINES 4 Sheets-Sheet 4.

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b r w to; NQ/Q QE 1 2 United States Patent 3,244,335 METHOD FOR FGRMENG SEVERANCE LINES Robert H. Downie, Neenah, Wis., assignor to American gain Company, New York, N.Y., a corporation of New ersey Filed Jan. 14, 1964, Ser. No. 337,677 8 Claims. (Cl. 225-1) This application is a continuation-in-part of application Serial No. 216,452, filed August 13, 1962, now Patent No. 3,170,342, which, in turn, is a continuation-in-part of application Serial No. 96,872, filed March 20, 1961, and now abandoned. This invention relates to new and improved cutting dies and particularly to a new method for forming a out line or out line pattern in a variety of sheet materials. The invention is of particular utility in the formation of paperboard carton blanks.

For many years, the cutting and scoring dies commonly used for the formation of carton blanks from paperboard have consisted of a series of individual cutting and scoring rules, or knives, hand formed to predetermined contours and held in frictional engagement between individually hand out wooden blocks commonly referred to as furniture. The assembly of knives, rules and wooden blocks is locked in position on a press bed plate and the opposing press member, which may be a back up plate or cylinder, bearing a covering of a hard, heavy counter paper, is brought into contact with the cutting and scoring die, thereby making on the counter paper and impression of the cutting and scoring rules in their proper spaced relationship. The counter paper is then manually cut away in the areas where the cutting and creasing rules registered an impression, thereby forming, in effect, a female counterpart of the male cutting and scoring rules in the counter paper. This system for the use of furniture dies in the production of carton blanks of paperboard has been in standard use in the industry for many years and is described in US. Patent 259,416, issued June 13, 1882 to E. B. and H. S. Munson.

Due to the many hand operations necessary for the preparation, assembly and corking of the knives, scoring rules and the wooden furniture of the male die and also the manual preparation of the female counter pa er counterpart thereof, this conventional die preparation and the press make-ready are extremely laborious, time-consuming and expensive. Furthermore, the accuracy with which carton blanks may be cut and scored with a die of this type depends equally on the skill and artistry with which the individual rules and knives are prepared and bent and the indiivdual wooden furniture blocks are cut and fitted to form the male die, and the care with which the counter paper is manually grooved to co-act with the knives and rules of the male die. Since each piece of each die is individually and manually constructed, exact reproducibility between any two dies would be rare, indeed. The multitude of separate pieces, held together frictionally in a frame, are subject to various distortional stresses on the press and frequently become misaligned, resulting in unsatisfactory cutting and scoring of the paperboard stock and, on occasion, the entire die pops from the restraining frame and separates into its individual component parts, necessitating laborious re-assembly and press make-ready.

In the present invention, the assembly of individual blocks and rules constituting the male cutting and creasing die is replaced by a one-piece die member bearing in extremely shallow relief on its upper surface a photographically registered representation of the male scoring and cutting elements, the main surface of the member having been chemically etched away so that the scoring and cutting elements stand in slight relief. A complementary one-piece female die member is also provided which bears See cutting elements and scoring elements which co-act with those of the male member to form the scores and cut-lines of the finished carton blank. These elements are also photo-composed and in very shallow relief to the chemically etched background surface of the die member in much the same manner as previously mentioned regarding the male counterpart. Due to the extremely shallow relief, Which may range from 0.005 in. to about 0.035 inch, the corking procedure necessary with conventional dies is completely eliminated and the sheet material stock is gripped and maintained planar during the cutting and scoring operations. The die members of the present invention are adaptable for use on sheet material cutting and creasing presses of the flat bed type, and on those having a flat bed with a rotating counter member as well as on presses in which both operating members rotate, as will be explained more fully hereinafter.

The principles of the present invention may be best understood from the following specification and accompanying drawings, in which:

FIGURE 1 is a plan view of a photographic positive to be used in an exemplary method for the preparation of a male die member according to this invention,

FIGURE 2 is a plan view of a photographic positive to be used in the preparation of a complementary female die member,

FIGURE 3a and FIGURE 3b are enlarged perspective views of concomitant areas of finished male and female die plates, respectively, showing the integral raised cutting and scoring elements in very shallow relief above the etched plate background,

FIGURE 4 is a fragmental view of a portion of a rotary press with male and female die members according to this invention atttached thereto in operating relationship and illustrated in the operation of forming cut and score lines in a sheet material,

FIGURE 5 is an exploded perspective view, showing the pair of die plates of FIGURES 3a and 3b, partially cut away, arranged for operation on a sheet of paperboard placed there-between and showing the resulting cutting and scoring of the paperboard,

FIGURE 6 is a greatly magnified crosssection view of a portion of male and female plates in register showing both a male and a female scoring element, and a concomitant pair of cutting members,

FIGURE 6a is a modified view of a portion of FIG- URE 6 showing a pair of cutting members and a pair of creasing members, and

FIGURE 7 is a schematic view of an apparatus including cutting elements operating according to the principles of this invention.

The new and improved cutting and creasing dies of this invention are prepared according to one embodiment thereof by chemically etching two metallic plates bearing photocomposed images of complementary cutting and scoring elements in the form of a resist which prevents the etchant from attacking the metal in the image areas. For convenience, the two plates which together compose a cutting and scoring die will be herein designated as the male and female plates, respectively. The plates bearing the resist coating in the areas corresponding to the cutting and scoring elements of the dies may be prepared in a number of ways. For example, a master plan view drawing of the cutting and scoring elements for the male and female plates as they are to appear in the finished die plate may be made and photographically imposed on the previously photosensitized metallic plates by known pro cedures. The master plan view and the corresponding resist image formed therefrom will normally contain corrective factors which allow for undercutting during the etching process, the degree of such undercutting being predeterminable for specific etchable metals. If desired,

the master plan view may be prepared as an enlargement of perhaps 2 to times the size of the final desired view and then subjected to a photographic reduction in size before imposing the images on the metallic plates to be etched. This procedure minimizes the eflect of minor inaccuracies in the preparation of the magnified plan view. The plates subsequently may be etched after removal of the photosensitive resist from those areas of the plate not bearing the line images of the cutting and scoring elements.

The die plates may be made of any of a variety of etchable metals such as magnesium, copper, steel, bronze and the like, the choice of the particular metal to be utilized in a given case being determined by the relative importance of various factors such as durability, rapidity and ease of etching, flexibility, economy and other similar factors. The etching medium used will depend on the particular metal which is to be etched, satisfactory etchants for each of the metals listed as well as for other etchable metals being well known in the art. In general, magnesium is most often etched in an etchant bath composition based primarily on nitric acid while copper, steel and bronze are customarily etched with ferric chloride etchant compositions, although ammonium persulfate and other oxidizing etchants may also be used.

Similarly, the die plates may be of a photopolymerizable plastic composition such as photosensitized nylon or a photopolymerizable composition of a divinyl ester of polyethylene glycol or the like. The utilization of photopolymerizable plastic materials for the preparation of the die plates of this invention involves certain modifications of the basic process as applied to metals, such modifications being obvious to those skilled in the art.

A satisfactory method for preparing the new and improved cutting and scoring dies includes the formation of a photographic positive of the cut line and score line elements of the male die member in predetermined spaced relationship to one another as shown in FIGURE 1 and in such manner that the photographic representation of each score line is, after allowance for the corrective factor necessitated by anticipated undercutting during etching, of the exact width desired for the finished male scoring element. A corresponding photographic positive of the female die elements as shown in FIGURE 2 is similarly prepared. These photocomposed positives are preferably prepared on a dimensionally stable transparent base such as glass or a stable flexible base such as a polyester film, the base bearing on one surface coating of a photographic emulsion. The positive images of the die elements are subsequently transferred to the metal die plates.

The resulting, completely photocomposed photographic images of the male and female cutting and scoring elements are separately photographically transferred through the medium of suitable negatives, to separate sheets of a suitable etchable metal bearing a coating of a photosensitive resist. The exposed metal sheets are then developed and fixed in conventional manner and the re- 'sulting sheets, bearing a protective resist coating only in those areas corresponding to the cutting and scoring elements, are etched in a suitable etchant bath to adepth ranging from 0.005 to 0.035 inch depending on the stock to be cut and creased. The finished male and female die sheets will then bear the cutting and scoring elements in very shallow relief on the thin, flexible metallic plates. If the basic material constituting the die plate is a photopolymerizable plastic such as nylon or a polyethylene glycol divinyl ester composition, for example, the photocomposed image of the cutting and scoring elements is transferred through the medium of suitable negatives directly onto the surface of the photosensitive plastic sheet which is then developed and hardened by conventional processes for treating the particular photopolymerizable plastic material utilized.

As represented in FIGURES 3a and 3b, which show concomitant sections of finished male and female plates,

respectively, the male die plate, designated generally by the number 101m, has integral raised portions constituting male scoring elements 161, 162 and 142 and cutting members 151, 193, 194, 152, 171, 176, 185 and 186 in shallow relief above the etched background 102m. Plate 101m also bears registration apertures 103m mechanically drilled in the plate at points corresponding to previously photographically imposed register marks. Similarly, the female die plate 101] bears paired female scoring elements 161a and 161b, 162a and 162b, and 142a and 142b as well as cutting members 151a, 193a, 194a, 152a, 171a, 176a, 185a and 186a and register apertures 103 drilled in the plate at points corresponding to previously photographically imposed register marks. Upon inversion of the female plate 101 into superposition on male plate 101m with respective paired marks 103m and 103 (and similar pairs of register apertures in other sections of the plates) in register, the male and female scoring elements and the respective pairs of cutting members will be positioned for cutting and scoring operations in conjunction with each other.

The plates may be secured to opposed press members in registered position by adhesives, clamps, screws or other suitable means. In use with presses of the flat bed type, the thickness of the plates is not a factor of consequence, since the plates are held planar. In use on cylinder presses, it is preferable that at least the plate being attached to the cylinder be of near minimum thickness to increase its flexibility.

In rotary presses as shown in FIGURE 4, wherein both press members a and 1001) are cylindrical, it is preferable that both die members 101m and 101 be sufficiently thin and flexible to enable the dies to be readily bent into intimate conformity with the cylindrical press member surfaces and be secured thereto with suitable clamping or fastening devices such as the stud bolts and 106. Thus, in this application to rotary press operations, thin, flexible metallic die plates of equal overall thickness between about 0.013 inch to about 0.04 inch and etched to equal depths are preferred. These thin plates may, of course, be etched as flat plates and subsequently attached to the press cylinders in wraparound fashion, or may be etched in the curved condition on suitable equipment. If desired, matched die members of greater thickness may be pro-curved and subsequently etched, or the die element images may be photocomposed directly on the surfaces of matched cylinders, which may then be etched to form the die members in shallow relief as integral elements of the cylinder surfaces. As illustrated in preferred form in FIGURE 4, thin, flexible die plates are clamped or bolted to the surface of the cylinders of a rotary press and serve thereby to form cut and score lines in a sheet material carried into the operating area between the two die members 1011f and 101m by means of a releasable sheet material clamp 108.

In contrast to the conventional carton cutting and scormg die in which the cutting and scoring rules are separate units maintained in their proper spaced relationship only by frictional engagement with the intervening furniture, the cutting and scoring elements of the dies of the present invention are integral parts of the die plate itself. Every element of the plate is proportioned and positioned with the extreme accuracy possible in photographic reproduction and is permanently constrained to a position of perfect register with concomitant elements. This has, for the first time, made possible the preparation of cutting and scoring dies in which the dimensions and spacial relationships of the various cutting and scoring elements may be precisely tailored to obtain the optimum efficiency in cutting and scoring operations for the particular type and caliper of sheet material stock to be worked. Furthermore, since the die elements are photographically composed, any number of die plates may be produced which are absolutely identical. This perfect reproducibility of the die plates constitutes a very important advantage of this invention.

Referring to FIGURES 6 and 6a, the vertical gap, v, between the plates '(that is, the vertical distance between the raised portion of the male die plate and the raised portion of the female die plate when the press is on impression) exerts considerable influence on the operation of the cutting elements. A further, closely related factor is the horizontal gap, g, or lateral distance between the co-acting cutting edges 181 and 182 of the pair of cutting members, 171 and 171a, one of which is integral with the male die plate and the other integral with the female die plate.

It has been found that in order to obtain satisfactorily clean and accurate cut lines in paperboard stock, for example, using the cutting dies of this invention, it is necessary that the horizontal gap, g, between the cutting edges 1811. and 132. of the cutting members be no greater than about of the paperboard stock thickness, as shown in FIGURE 6, and quite satisfactory cut lines may be obtained if the cutting members overlap to a small degree, as shown in FIGURE 6a, which should not normally exceed about 50% of the paperboard stock thickness. That is, when paperboard of 0.010 inch thickness is to be cut and scored, the horizontal gap, g, between the cutting members, when in operating position in the press, should fall in the range between 0.005 inch (indicating a combined overlap of the cutting members of 0.005 inch) and +0002 inch. For cutting paperboard of 0.016 inch caliper, the horizontal gap should be be tween 0.008 and +0003 inch. The desired gap range for cutting 0.025 caliper paperboard will be between 0.012 and +0005 inch, and a corresponding relationship will be found for paperboard of other thicknesses.

The vertical gap, v, between the male and female die plates when the press is on impression may range from a slight overlap which results in true shear cutting to a positive gap of up to about 50% of'the thickness of the paperboard stock being cut. It is, of course, impossible to operate under the special condition wherein the cut ting elements overlap both horizontally and verticaliy so that both g and v have negative values. 7

The preferred combination of horizontal and vertical relationships between cutting members for producing a clean ,cut in paperboard of a variety of thicknesses combines a vertical gap of 0.003 to 0.005 inch with a horizontal overlap of 0.002 to 0.004 inch. That is, when the press is on impression, the vertical distance between the cutting members of the die is between 0.003 and 0.005 inch and the cutting members overlap horizontally by 0.002 to 0.004 inch, each cutting edge overlapping the actual desired line of cut by 0.001 to 0.002 inch.

The dies of this invention are of value in forming cut and/ or score lines in a wide variety of sheet materials, including paper, plastics, leather fabrics, thin sheets of metal, wood, cork and the like, and, for each of these materials, relationships between the sheet material thickness and the critical die dimensions similar to those expressed above may be developed. 'For each type of sheet material, the optimum operating relationship "between the various die component dimensions and the stock thickness may be readily determined by simple test procedures.

It will be noted that the cutting elements of the dies of this invention include a flat portion lying in a plane generally parallel to the plane of the sheet material to be cut, this portion being part of the original surface of the die blank. Lying generally perpendicular to this portion of the cutting element and intersecting it to form a cutting edge is a second portion formed as an upstanding wall of the etched out area of the die member. In the preferred method of forming a cut line by the cooperative action of two cutting elements formed one on each of the two die members, the cutting elements overlap slightly in the plane of the material to be cut so that the material is etfectively clamped between the flat portions of the cutting elements and subsequently fractured 6 along a line running from the cutting edge of one cutting element to the cutting edge of the other cutting ele ment through the material clamped there-between. The fracturing of the sheet occurs before the cutting elements contact each other, generally when there remains a vertical gap ranging up to about 0.005 inch between the op posed cutting elements. The vertical gap remaining when fracture of the sheet material occurs is, of course, related to the type and thickness of the sheet material.

The method of forming a cut line in a sheet material through the use of cutting elements which overlap very slightly, as described immediately above, may be utilized in a variety of apparatuses operating either on individual sheets or on roll stock of a sheet material. FIGURE 7 of the accompanying drawings is a schematic representation of an exemplary apparatus for such operations on roll stock using a reciprocating die movement. In the apparatus shown, a roll of sheet material stock, 190, is fed over a series of guide rolls 191, 192 and 193,. one or more tensioning rolls 194, and driven nip rolls, 195 and 1%, onto a cutting bed 197 having removably inset therein a cutting element 198. A cutting head 199 including an inset cutting element 200 is mounted so that the two cutting elements, 198 and 200, will co-act as a result of a reciprocating motion applied to the cutting head 199, thereby forming a cut line in the sheet material 190. Each of the cutting elements 198 and 200 includes a planar portion generally parallel to the plane of the sheet material and a second portion generally normal thereto, the intersection between these portions forming a cutting edge on each cutting element. The cutting elements are mounted to overlap slightly in the plane of the material to be cut, as previously described and shown in FIGURE 6a, and the reciprocating cutting head is mounted so that a positive vertical gap remains between the cutting elements at the point of their closest approach.

After the cutting operation has been performed, the cut sheet, which may be in the form of individual severed sheets 201 or as a continuous web bearing internal cut lines, is removed from the cutting area by a suitable conveyor system 202.

In order that the sheet material may be stationary during the severing operation, the nip rolls and 196 may be driven by an eccentric or other conventional mechanism for imparting intermittent motion. 'Register may be maintained by photoelectric device 203, or equivalent means.

Similarly, the method of forming a out line may be adapted to a rotary mechanism, the cutting elements being inset in opposed rotating cylinders in such manner that, at" the moment of forming a out line, the cutting elements are opposed in slightly overlapping relationship in the plane of the sheet material being cut and are vertically gapped by a few thousandths of an inch, the amount of the gap being related to the type and thickness of the material being cut as previously described. In such rotary apparatus, the nip rolls will be geared to impart continuous motion to the sheet material, rather than intermitten motion.

The die members of the present invention are particularly effective in the cutting and scoring both of uncoated paperboard and of paperboard coated with a variety of protective and decorative coatings, including lacquers, varnishes, polymeric materials such as the polyolefins and various polymerized vinyl compounds and the like.

The precise control of the dimensions of the various elements of the die plates and of the horizontal and vertical distance relationships between these elements which is attained through the practice of this invention has made possible the achievement of a unique and highly useful cutting technique applicable to a variety of coated sheet materials, including polymer coated paperboard. Under carefully controlled conditions within the ranges previously set forth, it is possible to sever, through the use of this invention, the paperboard base layer of a polymer coated paperboard sheet without disturbing the continuity of the coating layers. This phenomenon was hitherto considered impossible of achievement.

As. an example of the unique cutting effect achieved through this invention, a sheet of 15 point paperboard (0.015 inch in thickness) coated on each side with polyethylene of 0.002 inch thickness was subjected to the cutting action of a set of cutting die plates produced according to previously described procedures. The vertical gap, v, between the cutting elements was adjusted to 0.005 inch, or 26% of the overall thickness of the coated board. The horizontal gap, g, was -0.002 inch, indicating an overlap of two thousandths of an inch, or 10.5% of the overall coated board thickness. By the action of the cutting elements, a cut line was imposed in the paperboard sheet, but the polyethylene coating on each side of the sheet was unaffected by the cutting action of the die and the polyethylene film continuity was unbroken. The result, then, was a paperboard sheet bearing a cut line wherein the board was completely severed, while the polyethylene coating on each side of the sheet was continuous and unbroken, even in the area covering the line of cut in the paperboard sheet buried between the polyethylene layers. The pinch-cutting was achieved without distortion of the original surface contours of the coated board. That is, the main plane of the coated paperboard remained substantially unaltered by the pinch cut. This procedure presents a substantial contribution to the packaging art, since it allows the impression of cut lines or lines of weakness in a package wall for ease of opening without impairing the water, vapor, gas and greaseproofness of the package imparted by the coating. The advantage in being able to impress such weakness lines in the package wall after the coating operation will be obvious to those skilled in the packaging art.

The phenomenon of cutting the interior base layer of a coated paperboard without severing the exterior coating layers and while maintaining substantially unaltered the original surface continuity of the coated sheet is dependent not only on the precise adjustment of the spacial relationships between the cutting elements as achieved by the present invention but also on the strength and resilience characteristics and the thickness of the coating and on the impact rupture resistance and thickness of the basic paperboard layer. In order to sever the inner layer without forming an incision in the outer coating layer, the cutting action must, first of all, be of a pinch-cut type rather than a shear-cut or a blade-incision type of cut. The coating must have sufficient toughness and resilience to absorb the pinching impact force applied to the composite sheet and transfer the impact force to the inner paperboard layer. This layer, having a lower impact rupture resistance and therefore being more fractile than the coating, is then severed by the pinch-cutting force exerted by the dies, while the outer coating layers retain their continuity. The expression pinch-cutting force in the claims is intended to refer to this type of force as applied to a polymer-coated sheet material.

Among the coating materials having the desired properties of strength, resilience and toughness are the polyolefins such as polyethylene and polypropylene, polyesters, polyvinyl chloride, polyvinylidine chloride and coolymers of vinylidine chloride and acrylonitrile. Coatings of polyethylene are particularly satisfactory. Coatings of less than about 1 mil (0.001 inch) in thickness are generally not satisfactory for this purpose since they ten-d to rupture under the pinching pressure of the die elements, due to insuflicient cushioning ability to absorb the applied force, and the coating continuity is thereby lost. Coatings ranging from about 1 mil in thickness to upward of 4 mils have proven quite satisfactory, although coating thicknesses in excess of about 4 mils are relatively expensive and therefore in less general use than coatings in the l 'to 4 mil range, which are therefore preferred.

In general, the thicker coatings, due to their greater cushioning capacity, give greater latitude or a broader range of spacial relationships between the cuttingdie elements within which the particular pinch-cutting phenomenon hereinbefore described may be achieved. For example, the pinch cut, in which the inner paperboard layer is severed but the outer coating remains unruptured, may be obtained on paperboard of from about 0.013 to 0.016 in. thickness coated on each side with from 2 to 4 mils of polyethylene with various combinations of the die cutting elements previously described in which the horizontal gap may vary between +0002 in. and 0.008 in. and the vertical gap may-range from 0.003 in. and 0.007 in. Similar paperboard coated on each side with 1 mil of polyethylene can be pinch cut to leave the coating unruptured only within the narrow range of die conditions wherein the vertical gap may range between about 0.003 and 0.005 in. and the horizontal gap may range from about +0002 to 0.006 in. If the horizontal and vertical gaps are reduced beyond the lower limits stated, the coating layers will be severed as well as the inner layer of paperboard, and the coated board will be completely cut through as in a normal cutting operation.

In general, pinch-cutting may be achieved when the vertical gap, v (as shown in FIGURE 6), between the cutting elements of the die lies within the range from about 14% to about 35% of the overall thickness of the coated sheet material and the corresponding horizontal gap, g, varies between a gap of about 12% and an overlap of about 35% of the overall coated sheet thickness. Optimum conditions within these limits for a particular ratio of paperboard thickness ranging from about 0.012 in. to about 0.030 in. to coating thickness preferably in the range from 0.001 in. to about 0.004 in. may be determined by experimentation, bearing in mind that the limits for the pinch-cutting of sheets having relatively thin coatings are somewhat more critical than for the relatively heavier coated sheets.

The pinch cutting operation is not limited to use on coated paperboard. By use of the present invention, the pinch-cutting technique may be extended to include the cutting of the inner layer of a large number of laminated structures without severing the outer layers or altering the original surface continuity of the coated sheet, if the outer layers are composed of a resilient and somewhat elastic material having high impact rupture resistance While the inner layer is of a material having a relatively low elasticity, resilience and impact rupture resistance. Leather, fabrics or relatively brittle metals coated with tough, resilient and elastic coatings of rubber, polyethylene, polyvinylchloride or the like may be pinch-cut by the process of the invention without destroying the continuity of the coating layers. The pinch-cut operation may also be utilized to sever the paperboard sheet material layer of a one-side coated sheet without disrupting the continuity of the coating layer. Furthermore, the same technique may be utilized to sever the outside paperboard layers of a laminated sheet construction in which a resilient polymer such as polyethylene is sandwiched between two layers of paperboard. By proper application of the pinch-cut principle, the outer layers of the frangible sheet material may be severed without dis- {upting the continuity of the inner, resilient polymer ayer. The combined cutting and scoring die plates of my invention may be of varying thickness depending on the particular type of press on which they are to be utilized and on the caliper and character of the paperboard stock which is to be cut and scored.

The minimum practical thickness of the die plates is about 0.005 in. greater than the height of the scoring and cutting elements above the etched background. Thus, for use in cutting and creasing of 0.010 in. thick paper- 9 board, the overall die plate thickness may be as little as about 0.013 inch. For cutting and scoring of 0.017 in. thick paperboard, the overall die plate thickness may range upward from about 0.020 in. and plate thicknesses above about 0.026 in. are satisfactory for preparation of dies for cutting and scoring of 0.026 in. thick paperboard assuming in each case an etch depth of about 0.8 times the thickness of the paperboards being processed. Similar relationships exist for paperboard of other thicknesses. It will be noted that the cutting and scoring dies may be very much thinner and consequently more flexible than conventional cutting and scoring dies made from an assembly of individual rules and furniture blocks, a maximum plate thickness of 0.04 in. being ample to pro-v vide cut-ting and scoring elements of optimum dimensions for all normal operations on even very heavy paperboard stock. In general, it is desirable from the standpoints of economy and utility to use die plates of near-minimum thickness. The very thin plates of the invention are sufficiently light in weight to be adhesively secured to either fiat or cylindrical press members without the necessity of clamps or screws. The lightness and flexibility of these plates makes possible the unique method of press makeready, previously described, in which in-register positioning of the die plates is obtained under .the dynamic operating conditions of the press. This is of particular value in presses in which at least one of the main operating members is a rotating cylinder. Die plates which are to rest on flat surfaces may be of greater thickness, since flexibility is not a factor in these cases and it may, on occasion, be desirable to use plates of 0.5 in. to 1.0 in. or more in thickness or to build up the thickness of a thinner die plate by adhering it to a backing material of metal, plastic, Wood or the like. On presses which combine a flat bed plate member with a cylindrical member, it is often convenient to utilize one die plate of minimum or near-minimum thickness on the cylinder and a somewhat thicker matching die plate on the flat bed, where flexibility is not a factor.

Various other modifications of the die-forming process herein described will suggest themselves to those skilled in the art, and it is not intended that the scope of the invention be limited except as necessitated by the appended claims.

I claim:

1. The method of forming a line of severance of predetermined contour in a frangible base sheet material having on at least one surface thereof a tightly adherent coating of a continuous film of a polymeric plastic material, said frangible sheet material having an impact rupture resistance lower than that of said coating, which comprises interposing said coated sheet between matched die plates of unyielding material, each die plate having a cutting element thereon, said cutting elements being contoured to coact to form said line of severance, moving said die plates toward each other with said coated sheet interposed therebetween to contact each side of said coated sheet with an unyielding die plate surface thereby applying a pinch-cutting force by unyielding surfaces at a predetermined locus to said coated sheet to sever said frangible base sheet at said predetermined locus, while there remains a vertical gap between said cutting elements, thus severing the frangible base sheet only while substantially maintaining the initial surface continuity of said coating.

2. The method of claim 1, wherein said frangible sheet material is a paperboard sheet.

3. The method of claim 2, wherein said polymeric plastic coating material is polyethylene.

4. The method of claim 3, wherein said paperboard sheet has a thickness between 0.012 in. and 0.030 in. and is coated on each surface with a polyethylene film having a thickness between 0.001 in. and 0.004 in.

5. The method of claim 1, wherein the application of said pinch-cutting force comprises applying co-acting forces to opposite sides of said sheet in the vicinity of said predetermined locus, said co-acting forces each having an effective cutting edge the horizontal position of which with respect to each other lie in the range of being spaced apart by about 0.012 times the overall thickness of said coated sheet to being overlapped by about 0.35 times such thickness and the limit of closest vertical approach of said cutting edges lies within the range of 0.14 to 0.35 times the overall thickness of said coated sheet.

6. The method of claim 1, wherein said pinch-cutting force is applied by a die comprising matched co-acting male and female die plates of etchable metal, said male die plate having as an integral part on one surface thereof a raised portion having a cutting edge thereon which constitutes the first of a pair of cutting members which pair together comprise a cutting element corresponding to said proposed line of severance in said frangible sheet material, said female die plate having as an integral part on one surface thereof a raised portion having a cutting edge thereon which constitutes the second of said pair of cutting members, said cutting members being positioned on the respective plates so that the cutting edges lie adjacent to the proposed line of severance and said raised portions in the horizontal relation with respect to each other are positioned between a spacing of about 0.12 times the overall thickness of said coated sheet and an overlap of about 0.35 times such thickness.

7. The method of forming a line of severance in a sheet material by engaging it from opposite sides along a proposed line of severance with co-acting cutting elements, each of which elements has a first portion lying parallel to the plane of the sheet material and a second portion generally normal to the plane of the sheet material to form a cutting edge at the intersection of said portions, said engagement comprising the steps of: (a) moving said elements toward each other and into contact with the sheet material with the cutting edges thereof offset in the plane of the material on opposite sides of the proposed line of severance and overlapping within predetermined limits related to the thickness of the sheet material to be severed, thereby clamping the material between the overlapping first portions of the elements, (b) fracturing said sheet material with said elements by working it between said first portions to an extent exceeding the elastic limits of the material generally along a line extending between the cutting edges of the elements, and (c) discontinuing movement of said elements toward each other short of contact with each other.

8'. The method of forming a line of severance of predetermined contour in a frangible sheet material having on at least one surface thereof a tightly adherent coating of a continuous film of a polymeric plastic material, said frangible material having an impact rupture resistance lower than that of said coating, which comprises (a) interposing said sheet material bearing said coating between the opposed faces of a pair of matched die plates, each of said die plates having as an integral part on the working face thereof a raised area of which a first portion lies generally parallel to the coated sheet material and of which a second portion lies generally normal thereto to form a cutting edge at the intersection between said first and second portions, said cutting edges being positioned on the respective plates to coact to form a line of severance in a material interposed therebetween,

(b) positioning said coated sheet material so that the cutting edges on the paired plates lie adjacent the proposed line of severance and offset with respect to each other in the plane of the sheet material on opposite sides of the proposed line of severance,

(c) moving said die plates toward each other and into contact with the coated sheet material with said parallel portions of said raised areas coming into contact with opposite sides of the sheet whereby said sheet is effectively clamped between said parallel portions of said raised areas,

(d) continuing the movement of said die plates toward each other to result in an application of a pinchcutting force of sufiicient magnitude to eifect fracturing of said frangible sheet material by exceeding the impact rupture resistance thereof along a line extending between the cutting edges of said die plate raised portions,

(e) discontinuing the movement of said die plates to- Ward each other sufiiciently short of contact with each other to avoid exceeding the impact rupture resistance of said polymeric plastic coating,

(1) moving said di-e plates away from each other sufficiently to release all contact with said coated sheet material, and

(-g) removing said coated sheet material from interposition :between said die plates.

References Cited by the Examiner UNITED STATES PATENTS 2,305,276 12/1942 Rushmore 225104 X 2,307,909 l/l943 Avery 156-268 X 2,397,347 3/1946 Gruner 225-103 X 2,956,717 10/1960 Scharf 225] FOREIGN PATENTS 1,011,764 4/1952 ,France.

WILLIAM W. DYER, 111., Primary Examiner.

ANDREW R. JUHASZ, Examiner. 

1. THE METHOD OF FORMING A LINE OF SEVERANCE OF PREDETERMINED CONTOUR IN A FRANGIBLE BASE SHEET MATERIAL HAVING ON AT LEAST ONE SURFACE THEREOF A TIGHTLY ADHERENT COATING OF A CONTINUOUS FILM OF A POLYMERIC PLASTIC MATERIAL, SAID FRANGIBLE SHEET MATERIAL HAVING AN IMPACT RUPTURE RESISTANCE LOWER THAN THAT OF SAID COATING, WHICH COMPRISES INTERPOSING SAID COATED SHEET BETWEEN MATCHED DIE PLATES OF UNYIELDING MATERIAL, EACH DIE PLATE HAVING A CUTTING ELEMENT THEREON, SAID CUTTING ELEMENTS BEING CONTOURED TO COACT TO FORM SAID LINE OF SEVERANCE, MOVING SAID DIE PLATES TOWARD EACH OTHER WITH SAID COATED SHEET INTERPOSED THEREBETWEEN TO CONTACT EACH SIDE OF SAID 